Switching power supply circuit with stored charge removal switch

ABSTRACT

A switching power supply circuit comprising a main switching element, and a second switching element which is on-off driven by the output of the main switching element. The design is made such that when the main switching element is turned on, the second switching element is turned off, while when the main switching element is turned off, the second switching element is turned on, and unwanted charges stored at the main switching element can be discharged through the second switching element when the main switching element is turned off.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a switching power supply device which is usable as a DC power source for various types of electric apparatus and implements. More particularly, the present invention a switching power source circuit arrangement which is so designed that power loss is reduced and thus efficiency is enhanced.

2. Description of the Prior Art

Referring to FIG. 1 of the accompanying drawings, a pulse-width controlled type switching power supply circuit is shown as an example of the prior-art switching power supply circuit arrangement, wherein a main switching transistor Q₁ has its emitter connected to an input terminal 1 to which is applied a DC input voltage V_(i), the collector of the transistor Q₁ being grounded through a primary winding N₁ of a transformer T. Furthermore, the transistor Q₁ has its base connected to a pulse width control circuit 2 through a base-current limiting resistor R₁, and also to the input terminal 1 through a resistor R₂ which serves to prevent collector cutoff current from being caused to flow when the transistor Q₁ is rendered inoperative. Connected to a secondary winding N₂ of the transformer T is a rectifying--smoothing circuit comprising a diode D₁ and capacitor C₁, whereby a DC output voltage is applied to a load 3.

When the transistor Q₁ is turned on, a current is caused to flow from the DC power source connected to the input terminal 1 to the primary winding N₁ of the transformer T, and resultant power is stored therein in the form of magnetic energy. Subsequently, when the transistor Q₁ is turned off, the magnetic energy stored in the primary winding N₁ is caused to appear, in the form of a counterelectromotive force, at the secondary winding N₂, thus resulting in the diode D₁ being rendered operative so that power is supplied to the load 3.

Output voltage, divided by resistors R₃ and R₄, is derived and compared in an error amplifier 4 with a reference voltage available from a reference voltage source 5. As a result, a feedback voltage corresponding to the difference between the output voltage and the reference voltage is applied from the error amplifier 4 to the pulse-width controlling circuit 2. With the pulse-width controlling circuit 2, the pulse-width of a drive pulse to turn on the transistor Q₂ is determined on the basis of the feedback voltage derived from the error amplifier 4, and the drive pulse is imparted to the base of the transistor Q₁ through the resistor R₁. More specifically, when the output voltage decreases, the feed voltage also decreases so that drive pulse to increase the "on" period of the transistor Q₁ is imparted to the base thereof. Conversely, when the output voltage increases, the feedback voltage also builds up, so that a drive pulse to decrease the "on" period of the transistor Q₁ is derived from a voltage comparator 17.

It is usual that the design is made such that only the cutoff and saturation regions of the switching transistor Q₁ are utilized to minimize loss which tends to occur during the switching operation. To make the transistor Q₁ saturated, it is necessary to flow a sufficient base current to establish a so-called "over-drive" condition.

Let it be now assumed that the potential of the pulse derived from the pulse-width controlling circuit 2, i.e., potential occurring at a point A in FIG. 1, is at low level (i.e., nearly equal to 0 V) with respect to the input voltage V_(i). Under such a condition, the transistor Q₁ is turned on because of the base thereof being positively biased, so that a base current I_(b1) flows through the resistor R₁. The values for the resistors R₁ and R₂ are determined such that there flows such a sufficiently high base current I_(b1) to establish the over-drive condition of the transistor Q₁. As a result, a large quantity of charges are stored between the base and the emitter of the transistor Q₁. Thus, even when the potential at the point A is changed to high level (nearly equal to V_(i)), a period of time will be taken before the transistor Q₁ which has been conducting, is completely turned off. Collector current I_(c1) decreases gradually during the interval between when the transistor Q₁ begins to be driven toward "off" state and when it is completely turned off, and thus a counterelectromotive force begins to occur at the transformer T. Consequently, as shown in FIG. 2(a), voltage V_(CE) between the collector and the emitter of the transistor Q₁ varies with time, so that there is a period of time Δt that the collector-emitter voltage V_(CE) and the collector current I_(c1) occurs at the same time. The product of the voltage and current which occurs during the period of time Δt is termed "turn-off loss", which accounts for a large proportion of the total loss of the transistor Q₁.

With the conventional circuit arrangement of FIG. 1, to reduce such turn-off loss by reducing Δt, it is only required that the value for the resistor R₂ be reduced so as to permit of quick discharge of charges accumulated between the base and the emitter of the transistor Q₁. However, there is a tendency that the base bias voltage of the transistor Q₁ is changed if the value for R₁ alone is reduced; thus, the value for the resistor R₁ should also be reduced Disadvantageously, however, if both the value for the resistor R₁ and that for the resistor R₂ are reduced, then the base current I_(b1) of the transistor Q₁ will be further increased. As known in the art, the base current I_(b1) is a current needed to turn on the transistor Q₁ and does not contribute directly to conversion efficiency. Thus, the increase in the base current I_(b1) will lead to a decrease in the efficiency. As will be seen from the foregoing discussion, the prior-art arrangement is disadvantageous in that a certain amount of turn-off loss is inevitably caused since an attempt to reduce the value for the resistor R₁ has its limit.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a switching power supply circuit which is so designed that turn-off loss is greatly reduced by reducing the aforementioned Δt and the efficiency is remarkably enhanced by rendering the circuit operable even if the base current I_(b1) of a switching transistor is reduced.

According to an aspect of the present invention, there are provided a main switching element, and a second switching element which is on-off driven by the output of the main switching element. The design is made such that when the main switching element is turned on, the second switching element is turned off, while when the main switching element is turned off, the second switching element is turned on, and unwanted charges stored at the main switching element can be discharged through the second switching element when the main switching element is turned off.

Other objects, features and advantages of the present invention will become apparent from the ensuing description taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a conventional switching power supply circuit.

FIG. 2(a) is a waveform view illustrating the voltage-current characteristics of a main transistor provided in the conventional circuit arrangement.

FIG. 2(b) is a waveform view illustrating the voltage-current characteristics of a main transistor provided in the circuit embodying the present invention.

FIG. 3 is a circuit diagram showing an embodiment of the present invention.

FIG. 4 is a schematic view illustrating the main portion of the circuit shown in FIG. 3.

FIGS. 5 to 13 are circuit diagrams showing other various embodiments of the present invention respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, there is shown the switching power supply circuit according to an embodiment of the present invention, wherein parts corresponding to FIG. 1 are indicated by like reference numerals. A further explanation of such parts will be omitted.

As will be appreciated, the switching power supply circuit of this embodiment is characterized in that there are provided a main switching transistor Q₁, and a second switching transistor Q₂ which is arranged to be on-off driven by the output of the main switching transistor Q₁.

The transistor Q₂ has its emitter connected to an input terminal 1, the collector thereof being coupled to the base of the main transistor Q₁. The base of the transistor Q₂ is also connected to the input terminal 1 through a parallel circuit of a base-emitter protecting diode D₂ and collector cutoff current preventing resistor R₅, and coupled to a connection point between the collector of the main transistor Q₁ and a primary winding N₁ of a transformer T. The base of the main transistor Q₁ is connected to a pulse width controlling circuit 2 through a parallel circuit of a current limiting resistor R₁ and capacitor C₂. The capacitor C₂ serves to speed up the turning-on of the main transistor Q₁.

With the foregoing arrangement, let it now be assumed that the potential at a point A becomes zero level. Under such a condition, the transistor Q₁ is turned on because of its base being positively biased, so that base current I_(b1) is caused to flow through the parallel circuit of the resistor R₁ and capacitor C₁. When the transistor Q₁ is turned on, the collector potential thereof becomes substantially equal to the input voltage V_(i). Thus, the second transistor Q₂, which has its base connected to the collector of the main transistor Q₁ through a capacitor C₃ is not positively biased and hence turned off, having no effect on the turning-on operation of the main transistor Q₁ at all.

When the potential at the point A becomes high level, the transistor Q₁ is released from the positively biased state and thus turned off so that the flow of base current I_(b1) is interrupted. As mentioned above, a certain length of time occurs between the point of time when the main transistor Q₁ begins to be urged toward "off" state and the point of time when it is completely turned off, and the collector current I_(c1) gradually decreases down to zero, so that a gradually increasing counterelectromotive force is generated at the transformer T. As a result, the collector voltage of the main transistor Q₁ gradually becomes lower than the input voltage V_(i), and since the collector of the main transistor Q₁ is connected to one end of a capacitor C₃, a current I_(R5) which tends to charge the capacitor C₃, and base current I_(b2) begin to flow through the resistor R₅ and the base of transistor Q₂ respectively. Thus, the transistor Q₂ is turned on by the base current I_(b2), so that charges accumulated between the base and the emitter of the main transistor Q₁ are discharged at once. In this way, the main transistor Q₁ is abruptly urged toward "off" state, and as shown in FIG. 2(b), the Δt is greatly reduced so that the turn-off loss is reduced accordingly.

Referring to FIG. 4, the main portion of the switching power supply circuit according to this embodiment is conceptually illustrated, wherein the main transistor Q₁, and the second transistor Q₂ are shown symbolically as switches. In this figure, the two switches Q₁ and Q₂ are oppositely operated in interlocking relationship with each other so that when the switch Q₁ is turned on, the switch Q₂ is turned off while when the switch Q₁ is turned off, the switch Q₂ is turned on; and the output of the switch Q₁ constitutes a signal source for turning on and off the switch Q₂.

In the above-described embodiment, the signal for driving to turn on and off the second transistor Q₂ was taken out of the connection point between the collector of the main transistor Q₁ and the primary winding N₁ of the transformer T. However, it is only required that such a driving signal be derived from a signal source provided by the output of the main switch element, and various modifications will be possible with respect to the manner in which such a driving signal is taken out. Referring to FIGS. 5 to 8, there shown different arrangements for taking out a driving signal.

FIG. 5 shows a circuit arrangement in which a tertiary winding N₃ is provided on the transformer T, the tertiary winding being connected at one end to the emitter of the main transistor Q₁ and at the other end to the base of the transistor Q₂ through a capacitor C₃, and the remainder thereof is similar to FIG. 3. With this arrangement, too, an electromotive force at the primary winding N₁ and that at the tertiary winding N₃ occur in a time series and are directed in the same direction; thus, the same effect as that of the circuit arrangement shown in FIG. 3 can be produced.

FIG. 6 illustrates a circuit arrangement similar to that of FIG. 5, except that the tertiary winding N₃ is grounded at one end instead of being connected to the emitter of the main transistor Q₁. With this arrangement, too, an electromotive force at the primary winding N₁ and that at the tertiary winding N₃ occurs in a time series and are directed in the same direction, and thus it operates in a manner similar to the manner in which the circuit of FIG. 5 operates.

FIG. 7 shows a circuit arrangement in which NPN transistors are used as the main transistor Q₁ and second transistor Q₂ ; the main transistor Q₁ has its collector connected to the input terminal 1 through the primary winding N₁, the emitter thereof being grounded; and the second transistor Q₂ also has its emitted grounded, the base thereof being grounded through the parallel circuit of the resistor R₅ and diode D₂. In this arrangement, a signal for driving the second transistor Q₂ is taken out of the connection point between the collector of the main transistor Q₁ and the primary winding N₁ of the transformer T as in the embodiment of FIG. 3.

FIG. 8 illustrates a partly modified version of the circuit shown in FIG. 7, wherein a driving signal is taken out of the connection point between the secondary winding N₂ and the diode D₁.

Although in the foregoing embodiments, description has been made of the cases where ordinary transistors were used as switching elements, it goes without saying that such transistors may be replaced with MOS transistors.

Furthermore, it is possible that the second switching element may be constituted by a photocoupler.

Referring to FIGS. 9 to 13, there are shown examples in which the second switching element is constituted by a photocoupler 6 comprising a phototransistor Q₃ and light-emitting diode D₃.

FIG. 9 illustrates an arrangement in which the phototransistor Q₃ of the photocoupler 6 is connected between the input terminal 1 and the base of the main transistor Q₁ which is a PNP transistor; the light-emitting diode D₃ of the photocoupler 6 is connected at the anode thereof to the collector of the main transistor Q₁, the cathode of the light-emitting diode being connected in series with a protective diode D₄ for protecting the light-emitting diode D₃ from being damaged by an inverse voltage, and a current limiting resistor R₆, and grounded therethrough. A capacitor C₄, which is connected in parallel with the resistor R₆, serves as a speed-up capacitor but it may be omitted. The remaining portion of the arrangement shown in FIG. 9 is similar to FIG. 3

When the main transistor Q₁ is turned on, the light-emitting diode D₃ is provided with an inverse voltage and thus turned off. When the main transistor Q₁ is turned off, the light-emitting diode D₃ is rendered operative by a flyback voltage at the primary winding N₁ so as to emit light. Light emitted by the light-emitting diode D₃ is received by the phototransistor Q₃ so that the latter is turned on and as a result, charges accumulated between the base and the emitter of the main transistor Q₁ are discharged at once.

FIG. 10 shows an arrangement in which a tertiary winding N₃ is provided on a transformer T; a series circuit comprising a light-emitting diode D₃, protective diode D₄ and resistor R₆ is connected in series with the tertiary winding N₃ ; and a speed-up capacitor C₄ is connected in parallel with the resistor R₆. This circuit takes advantage of the fact the flyback voltage which occurs at the primary winding N₁ when the main transistor Q₁ is turned off, is induced in the tertiary winding N₃ which is magnetically coupled to the primary winding N₁. The light-emitting diode D₃ is connected to the transformer T in such a manner that it is forwardly biased and enabled to emit light when the main transistor Q₁ is turned off. In this way, the circuit arrangement of FIG. 10 is capable of producing similar effect to that of FIG. 9. The operation will remain unchanged even if the connection point between the light-emitting diode D₃ and the tertiary winding N₃ is coupled to the input terminal 1 as indicated by a broken line 7.

FIG. 11 shows an arrangement wherein a tertiary winding N₃ is provided on a transformer and grounded at one end; the tertiary winding N₃ is further grounded at the other end through a series circuit comprising a light-emitting diode D₃, diode D₄ and resistor R₆ ; and a speed-up capacitor C₄ is connected in parallel with the resistor R₆.

With this arrangement, too, only when the main transistor Q₁ is turned off, the light-emitting diode D₃ is made to emit light so that the phototransistor Q₃ is turned on, thus resulting in the turn-off loss being reduced.

FIG. 12 shows an arrangement using an NPN transistor as a main transistor Q₁ and being similar to the circuit arrangement of FIG. 9, wherein the main transistor Q₁ is grounded at the base thereof through a photocoupler 6 and phototransistor Q₃, the collector of the main transistor Q₁ being connected to an input terminal 1 through a primary winding N₁ of a transformer T. A series circuit comprising a light-emitting diode D₃, diode D₄ and resistor R₆ is connected across the primary winding N₁, and a speed-up capacitor C₄ is connected in parallel with the resistor R₆.

When the main transistor Q₁ is turned on, an inverse voltage is applied the light-emitting diode D₃ so that the latter is turned off. On the other hand, when the main transistor Q₁ is turned off, flyback voltage at the primary winding N₁ is applied to the light-emitting diode D₃ so that the latter is forwardly biased and caused to emit light, and the phototransistor Q₃ is turned off.

FIG. 13 shows a partially modified version of the circuit shown in FIG. 12, wherein a signal for driving the phototransistor Q₃ is taken out of the connection point between the secondary winding N₂ and the diode D₁.

As will be appreciated from the foregoing discussion, according to the present invention, charges stored accumulated between the base and the emitter of the main transistor are quickly discharged through the second transistor immediately after the main transistor is turned off. Thus, the Δt shown in FIG. 2 is greatly reduced so that turn-off loss is remarkably reduced.

Generally, the base current I_(b1) is a wasteful current that does not contribute to power conversion efficiency, and thus it is desirable that such a current be minimized. To this end, according to the present invention, the transistor Q₂ is utilized as an active resistor instead of the resistor R₂ of the prior-art circuit (FIG. 1), and the necessity to provide the resistor R₂ is eliminated. Thus, the value for the resistor R₁ can be determined from the actually required amount of base current I_(b1). In other words, according to the present invention, it is possible to eliminate such a problem with the prior art that the value for the resistor R₁ inevitably becomes low due to the necessity to make low the value for the resistor R₂ so as to cause charges accumulated at the main transistor Q₁ to be discharged rapidly, and thus it is possible to minimize the base current I_(b1).

As will be appreciated from the above discussion, according to the present invention, it is possible to greatly reduce turn-off loss and base current I_(b1) at the same time, thereby achieving a remarkably enhanced power conversion efficiency. The result of experiments conducted by using a multi-output type flyback converter with input voltage 9.6 V, output voltage of 9 V, 15 V, 20 V and -8 V, and power output of 800 mW, indicated that with the prior art, the conversion efficiency was 77% while with the present invention, such efficiency was as high as 81%, i.e., an improvement as great as 9% over the prior art was achieved.

While the present invention has been illustrated and described with respect to specific embodiments thereof, it is to be understood that various changes and modifications will become possible within the scope of the appended claims. 

I claim:
 1. A switching power supply circuit wherein an input DC voltage is applied to an input terminal through an inductance element and on-off controlled by means of switching elements so that a pulse voltage is provided, and said pulse voltage is rectified and smoothed out to provide a predetermined DC output voltage, said switching power supply circuit comprising:(1) a transformer including at least a primary and a secondary winding, said primary winding being arranged to be provided with said input DC voltage; (2) a main switch element for on-off controlling said input DC voltage applied to said primary winding of said transformer and causing a pulse voltage to be generated at said secondary winding of said transformer; and (3) a second switch element arranged to be on-off driven by the output of said main switch element so that when said main switch element is turned on, said second switch element is turned off while when said main switch element is turned off, said second switch element is turned on, whereby unwanted charges accumulated at said main switch element are discharged through said second switch element when said main switch element is turned off.
 2. A switching power supply circuit according to claim 1, wherein each of said main second switch element is constituted by a PNP transistor having a base, an emitter and a collector; the base of said main switch element is connected to the collector of said second switch element; the emitters of said main and second switch elements are coupled to said input terminal; and the base of said second switch element is connected to said input terminal through a parallel circuit of a diode and resistor.
 3. A switching power supply circuit according to claim 2, wherein the base of said second switch element is connected to the collector of said main switch element through a capacitor.
 4. A switching power supply circuit according to claim 2, wherein said transformer is provided with a tertiary winding; said tertiary winding is connected at one end thereof to said input terminal; and the base of said second switch element is connected to the other end of said tertiary winding through a capacitor.
 5. A switching power supply circuit according to claim 2, wherein said transformer is provided with a tertiary winding, said tertiary winding is grounded at one end thereof; and the base of said second switch element is connected to the other end of said tertiary winding through a capacitor.
 6. A switching power supply circuit according to claim 1, wherein each of said main and second switch elements is constituted by an NPN transistor having a base, an emitter and a collector; the collector of said main switch element is connected to said input terminal through the primary winding of said transformer; the base of said main switch element is connected to the collector of said second switch element; the emitters of said main and second switch elements are grounded; and the base of said second switch element is grounded through a parallel circuit of a diode and resistor.
 7. A switching power supply circuit according to claim 6, wherein the base of said second switch element is connected to the collector of said main switch element through a capacitor.
 8. A switching power supply circuit according to claim 6, wherein the base of said second switch element is connected to the secondary winding of said transformer.
 9. A switching power supply circuit according to claim 1, wherein said main switch element is constituted by a PNP transistor having a base, an emitter and a collector; said second switch element is constituted by a photocoupler comprising a phototransistor and light-emitting diode; and said phototransistor is connected between the base of said main switch element and said input terminal.
 10. A switching power supply circuit according to claim 9, wherein the collector of said main switch element is grounded through a series circuit of a light-emitting diode, diode and resistor.
 11. A switching power supply circuit according to claim 9, wherein said transformer is provided with a tertiary winding; and a series circuit of a light-emitting diode, diode, and resistor is connected to said tertiary winding.
 12. A switching power supply circuit according to claim 9, wherein said transformer is provided with a tertiary winding grounded at one end thereof; and the other end of said tertiary winding is grounded through a series circuit of a light-emitting diode, diode and resistor.
 13. A switching power supply circuit according to claim 1, wherein said main switch element is constituted by an NPN transistor having a base, an emitter and a collector; the collector of said main switch element is connected to said input terminal through the primary winding of said transformer; said second swith element is constituted by a photocoupler comprising a phototransistor and light-emitting diode; and said phototransistor is connected between the base of said main switch element and the ground.
 14. A switching power supply circuit according to claim 13, wherein a series circuit of a light-emitting diode, diode and resistor is connected across the primary winding of said transformer.
 15. A switching power supply circuit according to claim 13, wherein a series circuit of light-emitting diode, diode and resistor is connected to one end of the secondary winding of said transformer. 